Early gold mining was confined to rich out-crops from which gold was recovered by simple wash techniques. With the discovery of pyritic ores, just before the turn of the century, it became necessary to separate yet smaller amounts of gold from even larger amounts of ore. In extracting the gold, typically the ore was contacted with mercury to recover a part of the gold, and gold not removed by mercury extraction was recovered in a cyanidation process. As the rich deposits became depleted, it became necessary to process leaner deposits. Moreover, it became necessary to process refractory gold ores, i.e., ores which are not amenable to conventional cyanidation treatment, often because of their excessive content of metallic sulfides (e.g., pyrite) or organic carbonaceous matter, or both. The pyrite tends to occlude finely disseminated gold contained in the ores, and organic carbonaceous impurities are capable of adsorbing a gold-cyanide cyanide complex. New leaching techniques have been developed, but nonetheless it is rather difficult to extract gold in the quantities desirable from untreated carbonaceous and carbonaceous pyritic ores. Recoveries were poor, and valuable resources were wasted.
Biohydrometallurgical processes were developed, but this type of processing too is not without its problems. A fine grind of the ore was required for satisfactory treatment, and it was necessary to use low solids slurries. Moreover, long treatment times were required.
In a cyanidation process, which remains in use today, a crushed, particulate ore is contacted and sold dissolved by a dilute solution of an alkali or alkaline-earth metal cyanide, e.g., calcium cyanide or sodium cyanide, to produce a gold cyanide complex. A finely divided zinc powder is then added to the gold containing cyanide solution, the zinc replacing the gold in the cyanide solution. The gold is precipitated as a black powder which is separated from the solution by filtration. The powder is mixed with fluxes, and then smelted. The impurities form a slag, leaving the gold behind as crude bullion from which, with further refining, pure gold is recovered.
Analysis of acidic waste water from various mines, e.g., copper mines, led to the discovery some years ago that the presence of soluble iron, copper and sulfuric acid was not due to a purely chemical process, but instead to a microbiological process in which autotrophic bacteria oxidized the iron, i.e., Thiobacillus ferrooxidans, and sulphur, i.e., Thiobacillus thiooxidans, to leach iron and copper from sulfide ores. The Thiobacillus ferrooxidans can obtain carbon for biosynthesis solely from CO.sub.2 fixation, and energy from the oxidation of Fe.sup.2+ to Fe.sup.3+ or from the oxidation of either elemental sulfur or reduced sulfur compounds. The oxidation of insoluble sulfur to sulfuric acid can also be performed by Thiobacillus thiooxidans. These organisms are often found in admixture on the examination of leaching dumps and both have been used in improved microbial leaching processes for the recovery of metals from ores. Moreover, since the development of these processes, other microbial leaching processes have been developed, and processes based on microbial leaching may include the older more conventional chemical treatment steps.
Vast quantities of ores are available throughout the world, inclusive of low grade ores and waste heaps produced as by-products during the mining and milling of high grade ores, and many of these ores and waste heaps contain strategic and precious metals, particularly gold. Gold is usually present only in minute concentration in any ore, e.g., pyritic ores, this in itself making its recovery difficult by conventional processes, e.g., cyanide treatment. It does not appear possible to recover all of the gold from any ore, often not even enough to make its recovery from an ore economically justifiable. Moreover, much gold can remain in an ore after treatment even though sufficient gold is recovered to make the process economically viable. This represents the waste of a valuable asset.
The necessity of reducing an ore to small particle size for biohydrometallurgical treatment is in itself burdensome. Many gold recovery processes in use today generally cannot efficiently process an ore without having to reduce the particle size to about 200 to 400 mesh Tyler. Moreover, in slurrying the ore for biohydrometallurgical processing, the ore solids content of the slurry generally cannot exceed about 10 percent, base on the weight of the slurry, for efficient processing. Further, the time required to efficiently process the ore generally exceeds size or seven days, and the amount of gold which can be recovered from the ore even over this period is less than desirable. Consequently, there is a need for more effective methods which would permit more efficient recovery of the gold at larger particle size, higher ore loadings in a given reactor space, reduced operating cycles, and higher gold recoveries. There is also a need for methods which can more efficiently process the more recalcitrant ores, particularly pyritic and carbonaceous ores.